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 Post subject: Selenium Rectifier Testing
PostPosted: Jan Sun 04, 2009 1:54 pm 
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Joined: Dec Tue 16, 2008 4:50 pm
Posts: 33
I have removed the selenium rect from my radio and replaced it with the 1N4007 diode. I have yet to power up the radio. I decided to check the old selenium rect, and found that it conducts in both directions with my meter set to ohms. It tested 1.7mohms in one direction and 3.7mohms with leads reversed. Is the sel/rect bad and would this cause abnormal (high)voltage to the filament string. I am trying to get up to speed so could someone explain. I know that the rect. should be a half wave rect I guess? Maybe?


MikeO


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 Post subject:
PostPosted: Jan Sun 04, 2009 2:44 pm 
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Joined: Feb Mon 04, 2008 3:20 am
Posts: 717
Location: Coopersville, MI 49404-9643
Hi Mike!!

You don't say what meter your using to check the rectifier, and that does make a difference because they don't all use the same voltage on higher scales. What you're seeing is normal. Check a diode and you'll see the same thing. When selenium rectifiers fail the voltage drop through them goes way high resulting in lower voltage to the set. Yes the rectifier is half wave. You don't say what set your working on, but it is possible that after the new diode is installed that the filament voltage may be (almost always is) higher than it should be so I would check that with the tubes out of the set. You can do some calculations (add the tube voltages together) to determine what the filament voltage should be at the first tube and go from there. If its a series string set, then it has a dropping resistor to get to the proper value of voltage and all the tubes should draw the same current. You can use the voltage drop of the tubes, add them together to determine total drop and then, using an older meter that loads the circuit, get a ballpart figure of voltage a t the first tube in the string. It should probably measure 15 to 20 percent high because of no load on the circuit. A better way is to look at the tube manual for current load of on of the tube filaments, figure the voltage drop of the entire tube string, and build a resistor that approximates the load and tack it into the circuit, fire up the set and measure the voltage present at the resistor. It should show the voltage required for the string of tubes. If its high, then adjust your dropping resistor value so that it's correct. Does this help??

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W8ZV
kim.herron@sbcglobal.net
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 Post subject:
PostPosted: Jan Sun 04, 2009 3:23 pm 
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Joined: Jun Wed 14, 2006 11:49 pm
Posts: 6130
Location: Leo, IN or Zellwood, FL
What he said sorta. Best Mike to come back and tell us what radio your working on. If it is a Zenith TO with the filament voltages developed from the cathode curcuits of the tubes. Then do NOT
try to power it up without tubes.
The Diode 1N4007 will give you a bit higher B+ voltage than a brand new Selenium Rectifier. In some radios you will want to add a series resistor to the diode to get this voltage back to correct number.

John k9uwa/w4 snowbird


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 Post subject: Selenium Rect. Testing
PostPosted: Jan Sun 04, 2009 4:35 pm 
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Joined: Dec Tue 16, 2008 4:50 pm
Posts: 33
The radio I am working on is a Zenith T/o Y600L. I was getting 38vdc at the first tube in the string, book states voltage should be between 8-9vdc. The meter I am using is a Fluke 73 series2 multimeter. I tested the diode and it tested the way I would think it should test. I took the first tube in the string out, powered up the radio to get a voltage reading. Based on what some members are saying I could have a problem when I reinstall the tube. Is this correct? Should I try to discharge the filter caps?

MikeO


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 Post subject:
PostPosted: Jan Sun 04, 2009 5:09 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
Yes, by all means discharge them before plugging the tubes back in! If you are only getting 38 volts with no load, then I would have to agree that the rectifier was bad.
Curt

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Curt, N7AH
(Connoisseur of the cold 807) CW forever!


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 Post subject: Re: Selenium Rect. Testing
PostPosted: Jan Sun 04, 2009 5:20 pm 
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Joined: Jun Wed 14, 2006 11:49 pm
Posts: 6130
Location: Leo, IN or Zellwood, FL
Don't RUN A ZENITH TO WITHOUT ALL THE TUBES IN IT.

Best also if you have one or get one... a 1R5 tube with a pin cut
off and put it in place of the 1L6 tube for all your testing.

See pricing of 1L6 tubes and you will understand why.

Voltages will NOT read correctly without all the stuff in place in this radio.

John k9uwa /w4


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 Post subject:
PostPosted: Jan Sun 04, 2009 6:01 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 34096
Location: Livermore, CA
MikeO

Do not operate your radio with a tube missing. Never pull or replace a tube in this radio with power on. You will blow at least one tube filament. Done it many times...

With tubes in the radio there is a load on the filament filter cap. This keeps voltage around 9. With a tube removed voltage will go way up across this cap. If you were to replace a tube with the cap charged one or more tube filaments will burn out.

Reading 38 volts shows a tube filament may be blown? You can check continuity of each tube filament by measuring resistance between pin 1 and 7. These are pins each side of the gap. Do this with tubes removed from the radio. Be sure the radio is unplugged when removing tubes.

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Norm


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 Post subject:
PostPosted: Jan Sun 04, 2009 7:15 pm 
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Joined: Dec Tue 16, 2008 4:50 pm
Posts: 33
I checked all tubes pins 1 and 7 and they all test good. I was told that the first number in the tube designation indicates the operating voltage for that tube. Is that correct? If that is true, I have 5 tubes in my filament string, 3V4,1U4,1L6,1U4,1U5. Doing the math I get 7vdc. The book calls for 8-9vdc at pin 7 of the first tube. I guess what you are saying is without a load this voltage would be much higher. Also, in testing voltages along the string, there should be a voltage drop across each tube plus any dropping resistors such that at the last tube the voltage should be 0. That seems to be what I am getting. Please forgive my stupidity, I'm not that bright but I work hard. Another thing, I am in the process of recapping this radio(Zenith T/O Y600L) and I have found all of the leads that are indicated by symbols but cannot find the leads to the 200uf cap which has no symbol. the filter can seems to only have 4 leads coming out. a 60uf,20uf,40uf and one unmarked lead which is brown in color which I took for the common ground for the caps or B-




MikeO


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 Post subject:
PostPosted: Jan Sun 04, 2009 7:18 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
The filament voltages designated by the first digit of the tube number are rounded off values. For the tubes starting with 1, the actual filament voltage is 1.4 volts, and for the tube with the 3 prefix, the actual voltage is 2.8 volts.
Curt

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Curt, N7AH
(Connoisseur of the cold 807) CW forever!


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 Post subject:
PostPosted: Jan Sun 04, 2009 7:24 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 34096
Location: Livermore, CA
MikeO

You are correct. The first number indicates filament voltage but instead of 1.5U5 they used 1U5. It's actually a 1.5 volt tube, like others starting with 1.

Each tube should have 1.4 - 1.5 volts across filament pins when operating in your radio. If it is mush less the oscillator (1L6) won't operate. If one tube reads real high. It's either bad or has poor socket contact.

Your filter capacitors are in a metal can. The metal can is common negative for all 4 sections. Symbols are shown on the schematic. The 200mf section isn't marked.

http://www.transoceanic.nostalgiaair.org/600/y600.pdf

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Norm


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 Post subject:
PostPosted: Jan Sun 04, 2009 7:38 pm 
Member

Joined: May Tue 30, 2006 4:46 pm
Posts: 7931
Location: Santa Rosa, CA
Norm: good advice.

MikeO, you might want to "google" for Transoceanic restoration tips. There are loads of useful pages. Here is one:

http://oak.cats.ohiou.edu/~postr/bapix/H500_2.htm

You can kill your tubes easily by high filament voltage or by changing them while the circuit is energized or the capacitors are charged.

Also, it's not clear that a VOM or DMM reading will tell you much about a selenium rectifier's condition. Unless the measurement circuit in the meter uses sufficient voltage to overcome the inherently high forward voltage drop of the selenium unit, it won't give a meaningful reading. The best way would be to measure the DC voltage across the selenium rectifier terminals with the rectifier out of the circuit and powered up by a small external power supply (maybe about 20 volts). A good selenium might be between 5 and 10 volts.

Changing to a silicon diode WILL increase the voltage on the capacitors and the tube filaments, unless you add a series resistor to compensate.

Rich


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 Post subject:
PostPosted: Jan Sun 04, 2009 8:21 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 9645
Location: Mpls, Minnesota
First of all you cannot test a selenium with a ohmeter, they must be tested under a specified load for voltage drop, the junction in a selenium does not form until current is applied. They are nothing like a silicon diode and cannot be tested the same way. Very seldom will you ever find a bad selenium in a TO.

Second, read everything you can on the restoration of TO's, you will save yourself a lot of trouble and blown tube filaments. The very first step is to replace the electrolytics and capacitors.

Dave


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 Post subject:
PostPosted: Jan Tue 06, 2009 9:29 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4799
Location: Woodinville, WA USA
I test selenium rectifiers by listening for the satisfying clang when I pitch them into the garbage can :)

Many people who restore TransOceanics add a power resistor (wirewound, etc.) in series with the new diode. Something rated for 2 or more watts, in the range of 47 to 180 ohms. If you are fussy, you can experiment with different values until your voltages look just right.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html


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 Post subject:
PostPosted: Jan Wed 07, 2009 12:31 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 34096
Location: Livermore, CA
Hi

A selenium rectifier is made up of several diodes. Each plate is one diode which are in series.

A diode has a specific voltage drop. All, usually 6 in series, add up to 7 volt drop on a good selenium rectifier. Meters use a 1 1/2 volt battery on ohm scale. This isn't enough to overcome the selenium rectifier drop. Some meters have a higher voltage battery on the meg ohm scale. Problem with this reverse direction of a selenium rectifier has enough leakage so there is a reading in either direction.

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 Post subject:
PostPosted: Jan Wed 07, 2009 2:52 am 
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Joined: Jun Wed 14, 2006 11:49 pm
Posts: 6130
Location: Leo, IN or Zellwood, FL
[quote="philsoldradios"]I test selenium rectifiers by listening for the satisfying clang when I pitch them into the garbage can :)

Phil Nelson

DITTO There is no such thing as a "good" Selenium Rectifier. Even a NOS one should be as Phil says.. Clanged into the Round File.

Besides they stink like crazy when croak.. plus the gas given off is claimed to... cause cancer.

John k9uwa /w4


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 Post subject:
PostPosted: Jan Wed 07, 2009 4:40 am 
Member

Joined: Oct Sun 07, 2007 5:22 am
Posts: 986
Location: Cascadia
Phil wrote
Quote:
I test selenium rectifiers by listening for the satisfying clang when I pitch them into the garbage can Smile


Yes, but they are part of the original radio's heritage. I never remove them if I can avoid it. On a radio or TV I plan to use a lot I might bypass the selenium rectifier and hide a modern diode under the chassis or elsewhere. But throw them away? Never.


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 Post subject: Re: Selenium Rectifier Replacement
PostPosted: Oct Sun 25, 2015 11:12 pm 
New Member

Joined: Aug Sat 29, 2015 12:16 am
Posts: 9
I have a Hallicrafter S-120 Shortwave Radio that I'm replacing parts in. It has a Selenium Rectifier in it. As many of you have pointed out,
it shows OPEN in either direction with a good quality DMM. It is wired up with a string of Tubes in series with 2 #47 Bulbs and a 500 Ohm 5W
resistor across the 2 #47 Bulbs. It then feeds a GloBar 880-100 to complete the full 120V Loop.

This all feeds a Globar 880-100, and that feeds a Selenium Rectifier, (shown as a single Diode) that, in series is connected to a 33 Ohm 5W Fuse
resistor; and the output of that feeds a 60uF 150V Electrolytic Cap. The schematic shows it should be at 130VDC. It reads 82V, and the radio
blew the 50C5 Tube. I've ordered 2 from Ebay.

I've read from a number of forums that I should use a 1N4007 and a resistor in series, in-place of the Se. (CR1)
Any feedback as to what size and wattage a series resistor should be for this circuit? I've read 200 Ohms at 10 Watts is a good choice.

Any feedback would be appreciated.

When I first got the S-120, it turned on, and made a lot of noise, but that was about it.
It originally had a 4-section Electrolytic. 60uF 150V -- 40uF 150V -- 40uF 150V -- 20uf 25V.
Some moron put 3 33uF caps. in for the first 3 sections.

I double-checked, NO, triple-checked to polarity of the caps I replaced. When I turned on the Radio, it blew the 50C5 immediately.

Since the .jpg won't fit because of size, you can get it at my OneDrive Short-Link: http://1drv.ms/1GBpCMb


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 Post subject: Re: Selenium Rectifier Testing
PostPosted: Oct Mon 26, 2015 12:18 am 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 7931
Location: Santa Rosa, CA
If the selenium reads "open" I am hard pressed to say what blew the 50C5. Seems like you have other problems.

There is no meaningful way to test a selenium with a DMM. The diode test setting won't give you a voltage high enough to read the forward voltage of the selenium (usually over 5 V and more on old, dying seleniums).

Rich


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 Post subject: Re: Selenium Rectifier Replacement
PostPosted: Oct Mon 26, 2015 12:27 am 
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User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 28280
Location: Maryland 20709, USA
pixster wrote:
Since the .jpg won't fit because of size, you can get it at my OneDrive Short-Link: http://1drv.ms/1GBpCMb
Hi pixster, and welcome aboard.
You might want to sign your posts so we know what to call you.

I don't think you'll find many folks here who will follow anonymous URLs. The chance of a problem is too high.

You might download Irfanview, the best photo manipulation program on the 'net, and it's free.
You can get it from Major Geeks: http://www.majorgeeks.com
I don't recommend any other source, including the originator's site, because he uses a hosting site that adds other files.

There are two files, the main one and the add-on for processing other file formats. Download both.
Install the main program first, then the add-on file.
-----
Both the schematic and the full manual for the S-120 are on BAMA, here:
http://bama.edebris.com/manuals/hallicra/s120/
-----
When you replace a selenium with a silicon (1N4007) you do not need to add a dropping resistor.
The difference in B+ is only 5 volts higher with the silicon diode.

You may want to add a dropping resistor if the set was originally designed for 110 volts AC, due to higher modern AC.
The rated max plate and screen voltages are 135v or 150v depending on which datasheet you believe.
Per the schematic, the plate and screen voltages in the S-120 are both 110v, so another 5 or 10 won't matter.

- Leigh

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73 de Leigh W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: Selenium Rectifier Testing
PostPosted: Oct Mon 26, 2015 8:13 am 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 1521
Location: Eastern Iowa
This article may be interesting to you:

http://w3hwj.com/index_files/RBSelenium2.pdf

You might want to Google search the considerations involved in changing over a tube-type ZTO from a selenium rectifier to silicon.
An unusual thing about the ZTO is that it uses the DC generated by the rectifier to not only create B+ for the set but also to operate the series filament string; so boosting the existing dropping resistor's value by 47 ohms (or by adding a new discrete resistor in series with the old one) is considered to be necessary to avoid overstressing the tubes.


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